1. Field of the Invention
This invention relates to an improved process for the preparation of aliphatic azo compounds. More specifically this invention relates to a process for the preparation of aliphatic azodinitrile compounds by reacting an aqueous hypochlorite solution with an aminonitrile.
2. Prior Art
The symmetrical azobis(alkanonitriles) and their derivatives have been recognized for over twenty-five years as useful, highly efficient free-radical polymerization initiators but the slow evolution of feasible and/or economical preparative routes have retarded the commercial development of these unique compounds.
The oldest method for producing aliphatic azonitriles commercially, described by Thiele and Heuser in Ann. 290, 1-43 (1896) is based upon the use of relatively expensive hydrazine and its derivatives as a principal raw material. Castle, U.S. Pat. No. 2,515,628, reacted ketones with sodium cyanide and hydrazine hydrochloride in aqueous medium and the resulting hydrazo compounds were oxidized in water or alcohol to the azos. Robertson, U.S. Pat. No. 2,586,995, prepared the ketazines from ketones and hydrazine hydrate, contacted them with liquid hydrogen cyanide to give hydrazo compounds and oxidized these to the azos. Both of the above procedures gave poor-to-fair yields of azonitriles. A significant process improvement in the hydrazine-based route was made by Koyanagi, et al., U.S. Pat. No. 3,775,395, who reacted a ketone, a hydrazine compound and hydrogen cyanide in aqueous medium in the presence of a surface active agent to form the hydrazo compound. Overall yields of azos were excellent.
A second method of producing azonitriles is described by Anderson, U.S. Pat. No. 2,711,405, which involves reacting the cyanohydrin of an aliphatic ketone with ammonia to form an aminonitrile and oxidatively coupling the aminonitrile to form the azo using an alkali metal or alkaline earth metal hypochlorite in aqueous medium. De Benneville, U.S. Pat. No. 2,713,576, claimed essentially the same process with the addition of alkyl hypochlorites and restriction of aminonitriles to those of acetone, methyl ethyl ketone and diethyl ketone. Although the process as disclosed in the Anderson and De Benneville patents is useful for the preparation of azobisisobutyronitrile from the aminonitrile of acetone, the coupling step results in extremely poor yields when applied to aminonitriles of higher molecular weight ketones. A process improvement which enables azonitriles to be prepared from aminonitriles of higher molecular weight ketones in good yields is reported by Fuchs, U.S. Pat. No. 3,783,148. Methanol or ethanol is employed as a reaction solvent in proportion to the amounts of aminonitrile and hypochlorite solution used such that, at the completion of the reaction, the alcohol concentration is at least 70% by volume. The alcohol maintains a homogeneous system throughout the reaction and specifically prevents separation of the intermediate, highly hydrophobic chloramines.
The Fuchs process, however, has its drawbacks. For economic reasons, the alcohol solvent must be recovered; even then, some loss of alcohol occurs due to evaporation, side reactions, etc. The alcohol also interacts with the hypochlorite compound, giving an organic alkyl hypochlorite, which can decompose in a highly exothermic reaction which imposes severe restrictions on the temperature and conditions under which the azonitrile preparation can be carried out.
The Fuchs process also has other disadvantages. In order to minimize the decomposition of alkyl hypochlorite and more easily control the heat load, the aminonitrile and hypochlorite are preferably added simultaneously. The molar ratio of hypochlorite to aminonitrile must be maintained in the range of 1.4 to 1.8 during the entire course of the addition. During the addition, the temperature must be held below -5.degree. C. Equipment and manpower requirements to maintain these stringent conditions are costly. Furthermore, use of other reaction conditions, such as the addition of the amino compound to the hypochlorite, which could enhance both yield and purity of the resulting azo compound, are obviated. In addition, product slurries with low solids content are obtained, because of the alcohol and water requirements to effect this process. This incurs considerable liquid handling and disposal per pound of product with unfavorable labor costs and production limits.
It is therefore desirable to discover a process which is free of the above disadvantages.